You could have alser with three energy levels involved.
The atoms are raised to the highest level, have a lasing transition to the middle level and then decay to the lower level with either the second transmission being a laser emmision or not.
I can't immediately think of any particular species but the energy level states for many gas lasers can be more complicated than the simple model.

Oh my god !!! guyz you didn't answer my question, and about mgb_phys answer the laser operation happened when electrons fall from the 2ND to the 1ST Level, not from the 3RD to the 2ND, and an example of it the ruby laser...
the decay happens from the 3RD to the 2ND level

Oh my god !!! guyz you didn't answer my question, and about mgb_phys answer the laser operation happened when electrons fall from the 2ND to the 1ST Level, not from the 3RD to the 2ND, and an example of it the ruby laser...
the decay happens from the 3RD to the 2ND level

Anyone got an answer? I mean for my FIRST QUESTION!!!!

Thats a little harsh don't you think, these people used there time to try to answer your question as best they could!

And the blame could be easily put on you for not explaining what the question is properly.

Ok now trying to answer your question.

The energy used to create population in version is high because you do not want to just emit a few photons, you need to excite a high number of electrons to the meta stable state (3rd level). You need this because you want to emit a large number of photons.

The 2 levels energy system is not possible to have a population inversion because the thermodynamical equilibrium will be reached when have n1=n2, while n is the number of atoms in the level,

3 levels uses a more clever way, it pumps energy to atoms to reach the last state (level 3), then the atoms decay very fast to a meta-stable state (level 2) which then have the stimulated emission and emit laser photons while the atoms are being lowered to first state

4 levels lasers is the best, and all these can be explained in wikipedia in the following link:

now lets talk about pumping power which i named (energy) but for sorry a misunderstanding happened

in 2 levels laser whatever the pumping was, the population inversion will never happen because the thermodynamical state will make n2=n1 in the best situation

in 3 levels laser when the pumping stats the pumped power must be greater than some amount for the population inversion to happen, which means the power must be BIG ENOUGH to achieve the population inversion and this was my question, physically WHY????

and the reason of the question that the 4 levels lasers starts the population inversion when starting the pump power

preciding is explained in my next post which shows in graphs when population inversion happens for each situation while the shaded part of the graph is the place we reach population inversion

If people do not stick to the question on hand but instead start making rather juvenile characterization of certain types of people, this thread will be deleted, the question will not be answered, and a few heads WILL roll merrily out of this forum.

Guyz, please stick on the question as ZapperZ said, I don't blame anybody, I don't deride of anybody, and remember we're from different countries and differrent languages and different habbits, thats why misunderstanding is very possible, I love scientific discussion with scientific people

They require more energy because you are trying to establish a population inversion with respect to the heavily populated ground state. To get a population inversion in a 3 level laser, you need to pump enough energy in to the system to elevate over 50% of atoms out of the ground state.

In contrast the active transition of a 4 level system is between some upper state and a lower state that is has an energy slightly above that of the ground level. In such systems you only need to elevate a small fraction (typically 1% or less) of the atoms above the ground state. This is because the lower energy state of the active transition has a low population - atoms tend to decay very quickly from this energy state into the ground state.

>theDestroyer
I'm supposed to know about lasers professionally but it's a few years since I had to actually care about energy levels rather than just which diode to use.
I was trying to remember the details of population inversion and why 2levels don't work - your post brought it all back.
I suppose I should have done some research before positing my original reply.